The present invention relates to the carbothermic reduction of alumina to produce aluminium metal.
There is described in U.S. Pat. No. 4,099,959 and in Pat. Applications Ser. Nos. 962,622, 962,630 and 962,652 a process in which a molten alumina slag containing dissolved aluminium carbide, travels successively through a zone of relatively low temperature, in which carbon feed material is added to the slag to react with alumina to augment the aluminium carbide content of the slag, and a zone of relatively high temperature in which aluminium carbide reacts with alumina to release aluminium metal which is collected and separated from the slag, the aluminium carbide content of the slag being simultaneously reduced.
The slag from the high temperature zone may be returned to the preceding low temperature zone in a 2-vessel system or it may be forwarded to a succeeding low temperature zone in a multi-vessel system.
The reaction in the low temperature zone may be represented as EQU 2Al.sub.2 O.sub.3 +9C Ai.sub.4 C.sub.3 (in solution)+6CO
whereas the reaction in the high temperature zone may be represented as EQU Al.sub.4 C.sub.3 (in solution)+Al.sub.2 O.sub.3 6Al+3CO.
These reactions are both highly endothermic and respectively proceed at temperatures within the ranges of about 1950.degree.-2050.degree. C. and about 2050.degree.-2150.degree. C. respectively.
The large volumes of gas released in the low temperature zone and in the high temperature zone carry substantial quantities of fume (both Al metal vapour and aluminium suboxide Al.sub.2 O). The amount of fume carried by the evolved CO is considerably greater in the gas evolved in the high temperature zone than in the gas from the low temperature zone because of the higher temperature. This is true whenever the carbothermic reduction of alumina is carried out in a system where the two above-mentioned reactions proceed in different zones of the system.
In U.S. Pat. No. 4,099,959 there is described an arrangement by which fume components can be removed from the evolved gas by passing the gas through the carbon feed material prior to introduction of the feed material to the low temperature zone.
It is an objective of the present invention to provide a simplified and more efficient procedure for the removal of fume components from the evolved gas. The method of the present invention is preferably used to complement the already described fume removal system and is particularly intended to achieve cooling of and partial fume removal from the gas evolved in the high temperature zone before subjection to the treatment described in U.S. Pat. No. 4,099,959.